In accordance with the present invention an "electroactive species" is intended to mean a chemical species, in the broadest sense, capable of undergoing changes in the oxidation state. This chemical species may be, for example, an atom, an ion, a molecule, a complex or a molecular system comprising ligands. In accordance with the invention and as described below, this species must be capable of being retained in one of its oxidation states on or in the working electrode by any suitable physical, chemical or physicochemical mechanism, for example by adsorption, amalgamation or the like.
A "liquid sample" is intended to mean any quantity of a liquid medium in which the electroactive, and especially ionized, species is present in trace form, regardless of whether the liquid medium to be analyzed is placed directly in contact with the working electrode or is diluted or mixed beforehand with a liquid matrix, especially a complexing solvent, and/or with an agent modifying the oxidation state of the electroactive species. Prior to the electroanalysis the liquid medium may or may not be divided up, so as to subject to the electroanalysis optionally a determined fraction of said medium.
"In trace form" is intended to mean molar concentrations per unit volume in the original liquid medium which are at most equal to 10 .mu.mol/l and preferably included according to the invention between 0.01 and 10 .mu.mol/l, or weight concentrations at most equal to 1 mg/l and included between 1 and 1000 .mu.g/l.
As stated already, the electroactive species must be capable of changing the oxidation state when a current or a potential is applied, preferably on electrical contact with a working electrode, while being deposited and being retained on the latter, by any suitable physical and/or chemical means such as the formation of an amalgam, dissolving in, or depositing on, a metal phase, for example gold, complexing, adsorption, absorption or else using electrostatic forces of attraction.
The electroactive species under consideration may be equally well cations, noble metals, anions, molecules or else complexes. Examples which will be mentioned are especially lead, cadmium, copper, bismuth, thallium, nickel, zinc, silver, platinum and halides as electroactive species capable of being detected and quantified according to the present invention.
Also, in accordance with the invention and as will be seen later, the electroactive species considered is capable of changing in oxidation state during the coulometric stripping, that is to say of being oxidized or reduced by means of a third element dissolved in the sample.
In order to make use of an electroanalysis according to the invention the electroactive species must pass through a number of oxidation states different from one another or some of them being the same ones, but the total number of electrons transferred during each stripping of the analysis must be known. These states are the following:
a zero or initial state (Eo) existing after preparation of the sample; PA1 a first state (EI), equal to or different from the initial state (Eo), existing after a main electrolysis or adsorption stage; PA1 a second state (EII), different from the first state (EI), different from or equal to the state (Eo), existing after a main coulometric stripping step; PA1 and a third state (EIII) different from the second state (EII), different from or equal to the state (EI) and existing after at least one post-electrolysis step followed by a coulometric poststripping. PA1 (a) during a main stage there is performed either a practically exhaustive electrolysis of the electroactive species, which is present in an initial state (Eo), at a predetermined potential, or a practically exhaustive complexing of said species with adsorption onto said electrode, while creating a convection regime in the vicinity of the liquid sample/working electrode interface, whereby practically all the electroactive species is deposited and retained, in a first oxidation state (EI), equal to or different from the initial state (EO), on the working electrode; if appropriate, the electrolysis and said convection regime are stopped; FIG. 3 thus shows the formation of a mercury amalgam in the form of film 18 and of electroactive species from a drop 17 deposited onto the electrode 16, after application of an electrolysis potential and of convection; PA1 (b) during a main coulometric stripping stage, represented by FIG. 4, a predetermined current is applied to the working electrode 16, and this causes a change in the oxidation state of the species (A), from the state (EI) to a second state (EII); this has the effect of detaching and diffusing practically all the electroactive species, in oxidized or reduced form from the working electrode 16 into the sample 17; the value of the predetermined oxidation or reduction current, in relation with the measured time of the coulometric stripping stage, makes it theoretically possible to determine the quantity, and hence the concentration, of the electroactive species originally present in the liquid sample 17. PA1 (c.a) during a post-electrolysis step a predetermined potential is applied to the working electrode 16 for a time which is shorter than that of the main stage (a), in this case, of electrolysis, account being taken of the ceasing of the convection regime existing during stage (a); this predetermined potential makes it possible to change the second oxidation state (EII) of the species (A) to a third state (EIII), in the case of at least one portion of said species which has diffused into the liquid sample 17 during the stripping stage (b), but which has not had the time to diffuse too far from the working electrode 16 (cf. FIG. 5); PA1 (c.b) during a poststripping step a predetermined current, strictly equal to the predetermined current of the main coulometric stripping stage (b) is applied to the working electrode 16, causing a change in the oxidation state of the species (A) from the state (EII) to the state (EIII), in order to detach and diffuse the electroactive species from the working electrode 16 into the liquid sample 17 (cf. FIG. 6). PA1 first of all essentially the multi-stripping described and defined above is used for the electroanalysis; PA1 the following operating conditions are then fixed: PA1 the electrolysis according to stage (a) is performed during a period at least equal to 4 times t1/2, t1/2 being the half-reaction time of the electrolysis of the electroactive species; this period ensures a virtually complete reduction or oxidation of the electroactive species; PA1 the main stripping current i.sub.ox1 is between 0.1 .mu.A and 500 .mu.A and preferably between 20 and 40 .mu.A; PA1 the electrolysis according to the stage (a) lasts several minutes, whereas the post-electrolysis of the coulometric poststripping cycle lasts only a few seconds; PA1 the current of another poststripping is approximately four times smaller than i.sub.ox1 ; PA1 the liquid sample analyzed additionally includes an agent modifying the electroactive and especially ionized species, promoting its electroanalysis; PA1 the electroactive species is especially an ionized metal and the liquid sample containing it also includes a molar excess of oxidizing agents, such as mercury II and/or gold III; PA1 the volume V of the liquid sample is at most equal to 100 .mu.l and preferably between and 10 and 50 .mu.l.
By way of preferred example of application of an electroanalysis according to the invention, but without any exclusiveness being implied, reference will be made to the investigation and to the quantification of heavy metals, in trace form, directly in a sample of whole blood, in the presence of dissolved and interfering oxygen.